This invention relates generally to improvements made upon high temperature aluminosilicate seal glasses containing CaO and BaO which are described and claimed in U.S. Pat. No. 4,060,423 and assigned to the assignee of the present invention. The present glasses maintain the metal oxide combinations within specified weight percent ranges as follows: SiO+Al.sub.2 O.sub.3 73-83, BaO+CaO 17-30 and with further ratios between BaO and CaO being maintained within the range 2.3 to 3.5, for greater reboil resistance. The term "reboil" as used herein is a tendency of the glass upon flame reworking or reheating to form many small bubbles of entrapped gases in the glass which produce poor light transmission as well as weaken the reheated areas. While small amounts of alkali metal oxides have been used in the past to reduce reboil, certain incandescent lamp types, such as the tungsten-halogen cycle lamp, cannot tolerate more than 300 ppm of a alkali metal oxide in the lamp glass envelope without experiencing operating difficulties. On the other hand, it is within contemplation of this invention to have 0-3 weight percent of an alkali metal ion in the present glasses for use in other incandescent lamp types requiring direct hermetic seals to tungsten and molybdenum components. For example, the present glasses can be substituted for the customary soda lime silicate glasses now providing the transparent envelope for conventional incandescent lamps having a resistive incandescent filament if higher lamp operating temperatures are required.
It has been discovered by the present applicant that the ratio of BaO to CaO in high temperature aluminosilicate glasses is critical for controlling reboil of the glass. Glasses having a ratio less than about 2.3 will tend to have some reboil on reheating and flame working of the glass, with the reboil becoming more severe as this ratio decreases. While increasing the SiO.sub.2 content in the glass can reduce reboil, such increased SiO.sub.2 content lowers the thermal expansion of the glass and for glasses to be useful in direct hermetic sealing to tungsten as well as molybdenum metals, an average thermal coefficient of expansion in the 0.degree.-300.degree. C. temperature range between about 37.times.10.sup.-7 cm/cm/.degree.C. to about 50.times.10.sup.-7 cm/cm/.degree.C. is required. The prior art glasses disclosed in the aforementioned issued U.S. patent have a thermal expansion coefficient suitable for hermetic seals only to metals with an expansion coefficient matching molybdenum. It is now possible by maintaining the BaO content at 10 percent by weight or greater and controlling the remaining metal oxides within critical proportions to achieve a proper match with metals having a lower expansion coefficient such as tungsten.
Specifically, the present invention relates to a novel hermetic seal means for an incandescent lamp utilizing the above identified improved seal glasses as the transparent lamp envelope and wherein the inlead construction of said lamp has a particular configuration. In said improvement, the pair of conductive inleads which are customarily connected to each resistive incandescent filament providing the light source in said lamp are further joined to relatively larger diameter inleads in the hermetic seal region which serve as adequate mechanical support means for the entire lamp. Joinder of the different size inleads at the hermetic seal region permits reliable hermetic sealing only to the smaller diameter inleads since it is not required that the larger diameter inleads extending therefrom and projecting outwardly from the lamp glass envelope also be hermetically sealed to the glass. Since the hermetic sealing of the larger diameter inleads has been obviated in this manner, there is no longer need for said larger diameter inleads to exhibit thermal expansion characteristics suitable for direct sealing to the lamp glass envelope and less expensive metals than tungsten or molybdenum can be employed for said components. Accordingly, known iron alloys including Kovar type alloys and other conductive metals now used as inlead materials in lamp construction can serve as the larger diameter inleads for the presently improved lamp.